1. Technical Field of the Invention
The invention relates to a corrective for eliminating the third order aperture aberration and the first order, first-degree axial chromatic aberration, comprising two correction pieces, which are arranged one behind the other, each correction piece having a plurality of quadrupole fields and at least one octupole field, and each correction piece being symmetrically constructed with respect to its central plane.
2. Description of the Prior Art
Electron-optical imaging systems are used both for enlarging, such as in electron microscopy, and for diminishing, such as in electron-projection lithography. Compared with light-optical imaging systems, they have the advantage of a substantially higher resolution, which is the result of the much smaller wavelength of the imaging optical rays. Compared to light, electron-optical imaging systems offer a resolution improvement of a factor of about 104 depending on the acceleration voltage.
The guidance of electron beams for the purposes of imaging is performed by means of electrical and/or magnetic lenses. Such lens systems, depending on their construction and arrangement, are subject to extremely diverse imaging aberrations, which can be classified as follows:
Axial aberrations: These are the image aberrations that occur with the imaging of an axial point and are only dependent on the aperture angle of the ray emerging from the axial point. Extra-axial aberrations: They occur with the imaging of an extra-axial image point and are determined by the distance of the image point from the optical axis (and possibly additionally by the aperture angle.) Each image aberration which is determined solely by its distance from the optical axis is termed distortion.
Chromatic image aberrations: since the imaging particles are not monochromatic, that is to say have different velocities, chromatic aberrations occur, which in turn can be subdivided into axial and extra-axial aberrations and are correspondingly co-determined by the aperture angle and/or by the distance from the optical axis. The axial and extra-axial image aberration are summarized as geometrical image aberrations in order to distinguish them from chromatic image aberrations. Finally, the axial image aberrations, since they only depend on the aperture angle, are also known as aperture aberrations. The efficiency of high-resolution electron-optical systems, such as those used in electron microscopy, is limited by spherical aberration (i.e., third order aperture aberration) and the first order, first degree axial chromatic aberration of the objective lens. Considerable efforts have therefore been made to eliminate these aberrations with the aid of appropriate correctives. One of the most promising methods is to use correctives that are constructed from multipoles, in particular those that generate quadrupole and octupole fields.
A corrective of this generic type can be found in German Patent DE-A 101 59 308 from the same applicant, which discloses a double-symmetrical arrangement of quadrupole and octupole fields. A disadvantage of this arrangement can be seen in the fact that no octupole fields are provided for correction of the third-order extra-axial coma, and that the octupoles, by virtue of their arrangement, generate large coma-like fifth order image aberrations for correction of the aperture aberrations. The illustrated principle of aperature and chromatic aberration correction in astigmatic intermediate images has proved incompatible with the essential demand for a large number of image points, which is essential for maximum resolution transmission electron microscopy.